Abstract Digest #, 11.11.2014

Today's abstracts:

  • [Apj] "Line-Driven Winds Revisited in the Context of Be Stars: Omega-slow Solutions with High k Values" by Jessie Silaj, Michel Cure, Carol E. Jones
  • [SF2A] "A search for Vega-like fields in OB stars" by C. Neiner, C.P. Folsom, A. Blazere
  • [AJ] "Long-term Optical Observations of the Be/X-ray Binary X Per" by Hui Li, Jingzhi Yan, Jianeng Zhou, Qingzhong Liu
  • [A&A] "Dynamics of the radiative envelope of rapidly rotating stars: Effects of spin-down driven by mass loss" by M. Rieutord and A. Beth
  • [ApJ] "Analytical Solutions for Radiation-Driven Winds in Massive Stars. I. The fast regime" by I. Araya, M. Cure, and L. S. Cidale
  • [AJ] "The Halpha Profiles of Be Shell Stars" by Jessie Silaj et al.
  • [A&A] "The spectroscopic Hertzsprung-Russell diagram of Galactic massive stars" by N. Castro et al.
  • [MNRAS] "Oscillation modes in the rapidly rotating Slowly Pulsating B-type star mu Eridani" by J. Daszynska-Daszkiewicz et al.
  • [Conference] "Probing the Structure of the Accretion Region in a Sample of Magnetic Herbig Ae/Be Stars" by Pogodin M.A. et al
  • "Diagnosing Mass Flows Around Herbig Ae/Be Stars Using the He I 10830 Line" by P. Wilson Cauley, Christopher M. Johns-Krull
  • [A&A] "Magnetorotational instability in decretion disks of critically rotating stars and the outer structure of Be and Be/X-ray disks" by Jiri Krticka, Petr Kurfurst, Iva Krtickova
  • [ApJ] "On the formation of Be stars through binary interaction" by Yong Shao, Xiang-Dong Li
  • [ApJ] "Suzaku Monitoring of Hard X-ray Emission from Eta Carinae over a Single Binary Orbital Cycle" by Kenji Hamaguchi et al.
  • [ApJL] "Eta Carinae's 2014.6 Spectroscopic Event: The Extraordinary He II and N II Features" by Kris Davidson et al.
  • [Acta Astronomica] "The All Sky Automated Survey. The Catalog of Bright Variable Stars in the I-band, South of Declination +28" by Monika Sitek, Grzegorz Pojmanski
  • [BeStars2014] "Modelling observable properties of rapidly rotating stars" by Diego CastaƱeda, Robert G. Deupree, Jason P. Aufdenberg
  • [BeStars2014] "Discovery of a Centrifugal Magnetosphere Around the He-Strong Magnetic B1 Star ALS 3694" by Matt Shultz et al.
  • [BeStars2014] "Emission from the Centrifugal Magnetospheres of Magnetic B-type Stars" by Matt Shultz et al.

Line-Driven Winds Revisited in the Context of Be Stars: Omega-slow Solutions with High k Values

Jessie Silaj, Michel Cure, Carol E. Jones

The standard, or fast, solutions of m-CAK line-driven wind theory cannot account for slowly outflowing disks like the ones that surround Be stars. It has been previously shown that there exists another family of solutions --- the Omega-slow solutions --- that is characterized by much slower terminal velocities and higher mass-loss rates. We have solved the one-dimensional m-CAK hydrodynamical equation of rotating radiation-driven winds for this latter solution, starting from standard values of the line force parameters (alpha, k, and delta), and then systematically varying the values of alpha and k. Terminal velocities and mass-loss rates that are in good agreement with those found in Be stars are obtained from the solutions with lower alpha and higher k values. Furthermore, the equatorial densities of such solutions are comparable to those that are typically assumed in ad hoc models. For very high values of k, we find that the wind solutions exhibit a new kind of behavior.

Available at: arXiv:1411.1465

A search for Vega-like fields in OB stars

C. Neiner, C.P. Folsom, A. Blazere

Very weak magnetic fields (with a longitudinal component below 1 Gauss) have recently been discovered in the A star Vega as well as in a few Am stars. According to fossil field scenarios, such weak fields should also exist in more massive stars. In the framework of the ANR project Imagine, we have started to investigate the existence of this new class of very weakly magnetic stars among O and B stars thanks to ultra-deep spectropolarimetric observations. The first results and future plans are presented.

Available at: arXiv:1410.2755

Long-term Optical Observations of the Be/X-ray Binary X Per

Hui Li, Jingzhi Yan, Jianeng Zhou, Qingzhong Liu

We present the optical spectroscopic observations of X Per from 1999 to 2013 with the 2.16m telescope at Xinglong Station and the 2.4m telescope at Lijiang station, National Astronomical Observatories of China. Combining with the public optical photometric data, we find certain epochs of anti-correlations between the optical brightness and the intensity of the H{\alpha} and HeI 6678 lines, which may be attributed to the mass ejections from the Be star. Alternative explanations are however also possible. The variability of FeII 6317 line in the spectra of X Per might be also caused by the shocked waves formed after the mass ejections from the Be star. The X-ray activities of the system might also be connected with the mass ejection events from the Be star. When the ejected materials were transported from the surface of the Be star to the orbit of neutron star, an X-ray flare could be observed in its X-ray light curves. We use the neutron star as a probe to constrain the motion of the ejected material in the circumstellar disk. With the diffusion time of the ejected material from the surface of Be star to the orbit of neutron star, the viscosity parameter {\alpha} of the circumstellar disk is estimated to be 0.39 and 0.28 for the different time, indicating that the disk around Be star may be truncated by the neutron star at the 2:1 resonance radius and Type I X-ray outburst is unlikely to be observed in X Per.

Available at: arXiv:1408.3542

Dynamics of the radiative envelope of rapidly rotating stars: Effects of spin-down driven by mass loss

M. Rieutord and A. Beth

Aims: This paper aims at deciphering the dynamics of the envelope of a rotating star when some angular momentum loss due to mass loss is present. We especially wish to know when the spin-down flow forced by the mass loss supersedes the baroclinic flows that pervade the radiative envelope of rotating stars.
Methods. We consider a Boussinesq fluid enclosed in a rigid sphere whose flows are forced both by the baroclinic torque, the spin-down of an outer layer, and an outward mass flux. The spin-down forcing is idealized in two ways: either by a rigid layer that imposes its spinning down velocity at some interface or by a turbulent layer that imposes a stress at this same interface to the interior of the star.
Results. In the case where the layer is rigid and imposes its velocity, we find that, as the mass-loss rate increases, the flow inside the star shows two transitions: the meridional circulation associated with baroclinic flows is first replaced by its spin-down counterpart, while at much stronger mass-loss rates the baroclinic differential rotation is superseded by the spin-down differential rotation. When boundary conditions specify the stress instead of the velocity, we find just one transition as the mass-loss rate increases. Besides the two foregoing transitions, we find a third transition that separates an angular momentum flux dominated by stresses from an angular momentum flux dominated by advection. Thus, with this very simplified two-dimensional stellar model, we find three wind regimes: weak (or no wind), moderate, and strong. In the weak wind case, the flow in the radiative envelope is of baroclinic origin. In the moderate case, the circulation results from the spin-down while the differential rotation may either be of baroclinic or of spin-down origin, depending on the boundary conditions or more generally on the coupling between mass and angular momentum losses. For fast rotating stars, our model says that the moderate wind regime starts when mass loss is higher than ~ 10-11 M_sun/yr. In the strong wind case, the flow in the radiative envelope is mainly driven by angular momentum advection. This latter transition mass-loss rate depends on the mass and the rotation rate of the star, being around 10-8 M_sun/yr for a 3 M_sun ZAMS star rotating at 200 km.s-1 according to our model.

Available at: A&A 570, A42

Analytical Solutions for Radiation-Driven Winds in Massive Stars. I. The fast regime

I. Araya, M. Cure, and L. S. Cidale

Accurate mass-loss rate estimates are crucial keys in the study of wind properties of massive stars and for testing different evolutionary scenarios. From a theoretical point of view, this implies solving a complex set of differential equations in which the radiation field and the hydrodynamics are strongly coupled. The use of an analytical expression to represent the radiation force and the solution of the equation of motion has many advantages over numerical integrations. Therefore, in this work, we present an analytical expression as a solution of the equation of motion for radiation-driven winds in terms of the force multiplier parameters. This analytical expression is obtained by employing the line acceleration expression given by Villata and the methodology proposed by Mueller & Vink. On the other hand, we

Available at: ApJ 75, 81

The Halpha Profiles of Be Shell Stars

Jessie Silaj et al.

A new set of theoretical Halpha emission line profiles of Be stars has been computed using the code BERAY, which solves the transfer equation along a series of rays passing through the star+disk system, representing an improved treatment over earlier work done by the authors. The new profiles were compared with the previous work, and general trends (such as line profile shapes and correlations between line equivalent widths as a function of initial density rho_0 and power law index n) were recovered. Additionally, BERAY was employed to model the spectra of eight well-known Be shell stars. Some degeneracy was found in the choice of model parameters, highlighting the need to employ alternate observables to constrain the models. However, the inclination angle of the model seemed relatively insensitive to the choices of other parameters, and we show that, with our models, only a very small range of inclination angles can adequately reproduce the observations. Five of our eight targets were found to have inclination angles of 70deg or higher, and two more were found to have inclination angles of 67deg and 65deg. The observation of one target, 4 Aquilae, could only be reproduced by models created at an inclination angle of approximately 45deg.

Available at: AJ 795, 82

The spectroscopic Hertzsprung-Russell diagram of Galactic massive stars

N. Castro et al.

The distribution of stars in the Hertzsprung-Russell diagram narrates their evolutionary history and directly assesses their properties. Placing stars in this diagram however requires the knowledge of their distances and interstellar extinctions, which are often poorly known for Galactic stars. The spectroscopic Hertzsprung-Russell diagram (sHRD) tells similar evolutionary tales, but is independent of distance and extinction measurements. Based on spectroscopically derived effective temperatures and gravities of almost 600 stars, we derive for the first time the observational distribution of Galactic massive stars in the sHRD. While biases and statistical limitations in the data prevent detailed quantitative conclusions at this time, we see several clear qualitative trends. By comparing the observational sHRD with different state-of-the-art stellar evolutionary predictions, we conclude that convective core overshooting may be mass-dependent and, at high mass (>15 M_sun), stronger than previously thought. Furthermore, we find evidence for an empirical upper limit in the sHRD for stars with Teff between 10000 and 32000K and, a strikingly large number of objects below this line. This over-density may be due to inflation expanding envelopes in massive main-sequence stars near the Eddington limit.

Available at: A&A 570, L13

Oscillation modes in the rapidly rotating Slowly Pulsating B-type star mu Eridani

J. Daszynska-Daszkiewicz et al.

We present results of a search for identification of modes responsible for the six most significant frequency peaks detected in the rapidly rotating SPB star mu Eridani. All published and some unpublished photometric data are used in our new analysis. The mode identification is carried out with the method developed by Daszynska-Daszkiewicz et al. employing the phases and amplitudes from multi-band photometric data and relying on the traditional approximation for the treatment of oscillations in rotating stars. Models consistent with the observed mean parameters are considered. For the five frequency peaks, the candidates for the identifications are searched amongst unstable modes. In the case of the third frequency, which is an exact multiple of the orbital frequency, this condition is relaxed. The systematic search is continued up to a harmonic degree l=6. Determination of the angular numbers, (l,m), is done simultaneously with the rotation rate, Vrot, and the inclination angle, i, constrained by the spectroscopic data on the projected rotational velocity, Vrotsini, which is assumed constant. All the peaks may be accounted for with g-modes of high radial orders and the degrees l<=6. There are differences in some identifications between the models. For the two lowest--amplitude peaks the identifications are not unique. Nonetheless, the equatorial velocity is constrained to a narrow range of (135, 140) km/s. Our work presents the first application of the photometric method of mode identification in the framework of the traditional approximation and we believe that it opens a new promising direction in studies of SPB stars.

Available at: arXiv:1410.6283

Probing the Structure of the Accretion Region in a Sample of Magnetic Herbig Ae/Be Stars

Pogodin M.A. et al

We present the results of a study of the temporal behaviour of several diagnostic lines formed in the region of the accretion-disk/star interaction in the three magnetic Herbig Ae stars HD101412, HD104237, and HD190073. More than 100 spectra acquired with the ISAAC, X-shooter, and CRIRES spectrographs installed at the VLT-8m telescope (ESO, Chile), as well as at other observatories (OHP, Crimean AO) were analyzed. The spectroscopic data were obtained in the He I lambda10830, Pa gamma and He I lambda5876 lines. We found that the temporal behaviour of the diagnostic lines in the spectra of all program stars can be widely explained by a rotational modulation of the line profiles generated by a local accretion flow. This result is in good agreement with the predictions of the magnetospheric accretion model. For the first time, the rotation period of HD104237 (P_rot = 5.37+-0.03 days), as well as the inclination angle (i = 21+-4deg) were determined. Additional analysis of the HARPSpol spectra of HD104237 and HD190073, taken from the ESO archive, with the use of the SVD method shows that the magnetic field structure of HD190073 is likely more complex than a simple dipole and contains a circumstellar component. For the first time, the magnetic field of the secondary component of the binary system HD104237 was also detected (<B_z> = 128+-10G).

Available at: arXiv:1410.6463

Diagnosing Mass Flows Around Herbig Ae/Be Stars Using the He I 10830 Line

P. Wilson Cauley, Christopher M. Johns-Krull

We examine He I 10830 profile morphologies for a sample of 56 Herbig Ae/Be stars (HAEBES). We find significant differences between HAEBES and CTTSs in the statistics of both blue-shifted absorption (i.e. mass outflows) and red-shifted absorption features (i.e. mass infall or accretion). Our results suggest that, in general, Herbig Be (HBe) stars do not accrete material from their inner disks in the same manner as CTTSs, which are believed to accrete material via magnetospheric accretion, while Herbig Ae (HAe) stars generally show evidence for magnetospheric accretion. We find no evidence in our sample of narrow blue--shifted absorption features which are typical indicators of inner disk winds and are common in He I 10830 profiles of CTTSs. The lack of inner disk wind signatures in HAEBES, combined with the paucity of detected magnetic fields on these objects, suggests that accretion through large magnetospheres which truncate the disk several stellar radii above the surface is not as common for HAe and late-type HBe stars as it is for CTTSs. Instead, evidence is found for smaller magnetospheres in the maximum red-shifted absorption velocities in our HAEBE sample. These velocities are, on average, a smaller fraction of the system escape velocity than is found for CTTSs, suggesting accretion is taking place closer to the star. Smaller magnetospheres, and evidence for boundary layer accretion in HBe stars, may explain the less common occurrence of red--shifted absorption in HAEBES. Evidence is found that smaller magnetospheres may be less efficient at driving outflows compared to CTTS magnetospheres.

Available at: arXiv:1410.6964

Magnetorotational instability in decretion disks of critically rotating stars and the outer structure of Be and Be/X-ray disks

Jiri Krticka, Petr Kurfurst, Iva Krtickova

Evolutionary models of fast-rotating stars show that the stellar rotational velocity may approach the critical speed. Critically rotating stars cannot spin up more, therefore they lose their excess angular momentum through an equatorial outflowing disk. The radial extension of such disks is unknown, partly because we lack information about the radial variations of the viscosity. We study the magnetorotational instability, which is considered to be the origin of anomalous viscosity in outflowing disks. We used analytic calculations to study the stability of outflowing disks submerged in the magnetic field. The magnetorotational instability develops close to the star if the plasma parameter is large enough. At large radii the instability disappears in the region where the disk orbital velocity is roughly equal to the sound speed. The magnetorotational instability is a plausible source of anomalous viscosity in outflowing disks. This is also true in the region where the disk radial velocity approaches the sound speed. The disk sonic radius can therefore be roughly considered as an effective outer disk radius, although disk material may escape from the star to the insterstellar medium. The radial profile of the angular momentum-loss rate already flattens there, consequently, the disk mass-loss rate can be calculated with the sonic radius as the effective disk outer radius. We discuss a possible observation determination of the outer disk radius by using Be and Be/X-ray binaries.

Available at: arXiv:1410.7831

On the formation of Be stars through binary interaction

Yong Shao, Xiang-Dong Li

Be stars are rapidly rotating B type stars. The origin of their rapid rotation is not certain, but binary interaction remains to be a possibility. In this work we investigate the formation of Be stars resulting from mass transfer in binaries in the Galaxy. We calculate the binary evolution with both stars evolving simultaneously and consider different possible mass accretion histories for the accretor. From the calculated results we obtain the critical mass ratios qcr that determine the stability of mass transfer. We also numerically calculate the parameter lambda in common envelope evolution, and then incorporate both qcr and lambda into the population synthesis calculations. We present the predicted numbers and characteristics of Be stars in binary systems with different types of companions, including helium stars, white dwarfs, neutron stars, and black holes. We find that in Be/neutron star binaries the Be stars can have a lower limit of mass ~8M_sun if they are formed by stable (i.e., without the occurrence of common envelope evolution) and nonconservative mass transfer. We demonstrate that isolated Be stars may originate from both mergers of two main-sequence stars and disrupted Be binaries during the supernova explosions of the primary stars, but mergers seem to play a much more important role. Finally the fraction of Be stars which have involved binary interactions in all B type stars can be as high as ~13, implying that most of Be stars may result from binary interaction.

Available at: arXiv:1410.0100

Suzaku Monitoring of Hard X-ray Emission from Eta Carinae over a Single Binary Orbital Cycle

Kenji Hamaguchi

The Suzaku X-ray observatory monitored the supermassive binary system Eta Carinae 10 times during the whole 5.5 year orbital cycle between 2005-2011. This series of observations presents the first long-term monitoring of this enigmatic system in the extremely hard X-ray band between 15-40 keV. During most of the orbit, the 15-25 keV emission varied similarly to the 2-10 keV emission, indicating an origin in the hard energy tail of the kT ~4 keV wind-wind collision (WWC) plasma. However, the 15-25 keV emission declined only by a factor of 3 around periastron when the 2-10 keV emission dropped by two orders of magnitude due probably to an eclipse of the WWC plasma. The observed minimum in the 15-25 keV emission occurred after the 2-10 keV flux had already recovered by a factor of ~3. This may mean that the WWC activity was strong, but hidden behind the thick primary stellar wind during the eclipse. The 25-40 keV flux was rather constant through the orbital cycle, at the level measured with INTEGRAL in 2004. This result may suggest a connection of this flux component to the gamma-ray source detected in this field. The Helium-like Fe Kalpha line complex at ~6.7 keV became strongly distorted toward periastron as seen in the previous cycle. The 5-9 keV spectra can be reproduced well with a two-component spectral model, which includes plasma in collision equilibrium (CE) and a plasma in non-equilibrium ionization (NEI) with tau ~1e11 cm-3 s-1. The NEI plasma increases in importance toward periastron.

Available at: arXiv:1410.6171

Eta Carinae's 2014.6 Spectroscopic Event: The Extraordinary He II and N II Features

Kris Davidson et al.

Eta Carinae's spectroscopic events (periastron passages) in 2003, 2009, and 2014 showed a progressive evolution in several respects. He II 4687 and nearby N II multiplet 5have special significance, because they are excited in unusual ways that sample very soft X-rays and the ionizing UV radiation field (EUV). HST/STIS observations in 2014 show dramatic increases in both compared to the previous 2009.1 event. These results appear very consistent with a progressive decline in the primary wind density, proposed years ago on other grounds. If material falls onto the companion star near periastron, the accretion rate may now have become too low to suppress the EUV.

Available at: arXiv:1411.0695

The All Sky Automated Survey. The Catalog of Bright Variable Stars in the I-band, South of Declination +28

Monika Sitek, Grzegorz Pojmanski

This paper presents the results of our extensive search for the bright variable stars in approximately 30000 square degrees of the south sky in the I-band data collected by 9 deg x 9 deg camera of the All Sky Automated Survey between 2002 and 2009. Lists of over 27000 variable stars brighter than 9 mag at maximum light, with amplitudes ranging from 0.02 mag to 7 mag and variability time-scales from hours to years, as well as corresponding light curves are provided. Automated classification algorithm based on stellar properties (period, Fourier coefficients, 2MASS J, H, K, colors, ASAS V-band data) was used to roughly classify objects. Despite low spatial resolution of the ASAS data (~15 arcs) we cross-identified all objects with other available data sources. Coordinates of the most probable 2MASS counterparts are provided. 27705 stars brighter than I=9 mag were found to be variable, of which 7842 objects were detected to be variable for the first time. Brief statistics and discussion of the presented data is provided. All the photometric data is available over the Internet at this URL

Available at: arXiv:1411.1277

Modelling observable properties of rapidly rotating stars

Diego CastaƱeda, Robert G. Deupree, Jason P. Aufdenberg

To fully understand the Be star phenomenon, one must have a reasonable degree of knowledge about the star beneath the disk, which is often found to be rapidly rotating. Rapid rotation complicates modelling because fundamental properties like the stellar luminosity and effective temperature require knowledge of the angle of inclina- tion at which the star is observed. Furthermore our knowledge of the structure of rapidly rotating stars is on a less sure foundation than for non-rotating stars. The uncertainties in the inclination and the surface properties of a few rapidly rotating stars have been substantially reduced by interferometric observations over the last decade, and these stars can be used as tests of rotating stellar models, even if those stars themselves may not be Be stars. Vega, as an MK standard, is historically a very important star because it is used for calibration purposes. However, several studies have suggested that Vega is a rapidly rotating star viewed at a very low inclination angle, raising questions as to how well we really know its properties. Appropriate modelling has been challenging and there is still room for debate over the actual properties of Vega, as opposed to its observed properties. We have previously shown that under certain conditions both the stellar surface properties and the deduced surface properties scale from one model to another with the same surface shape. We used this scaling algorithm with realistic 2D models to compute high-resolution spectral energy distributions and interferometric visibilities to determine the best rotating model fit to Vega. Detailed comparisons between the computed and observed data will be presented.

Available at: arXiv:1411.1673

Discovery of a Centrifugal Magnetosphere Around the He-Strong Magnetic B1 Star ALS 3694

Matt Shultz et al.

We report the results of 6 nights of Canada-France-Hawaii Telescope spectropolarimetric ESPaDOnS observations of the He-strong, magnetic B1 type star ALS 3694. The longitudinal magnetic field is approximately 2 kG in all 6 observations, showing essentially no variation between nights. The Halpha line displays variable emission on all nights, peaking at high velocities (~3vsini). Given the presence of a strong (Bd>6 kG) magnetic field, and the similarity of the emission profile to that of other magnetic B-type stars, we interpret the emission as a consequence of a centrifugal magnetosphere.

Available at: arXiv:1411.2534

Emission from the Centrifugal Magnetospheres of Magnetic B-type Stars

Matt Shultz et al.

Approximately 10% of B-type stars possess strong magnetic fields, and of these, 25% host centrifugal magnetospheres (CMs) in which the radiative wind, magnetic field, and rotational support interact to form a dense circumstellar plasma visible in a variety of diagnostic lines. In this article we review the basic theory behind CMs, outline current theoretical and observational problems, compare the observational properties of CM host stars to those of classical Be stars, and finally present preliminary results of a population study aimed at clarifying the characteristics of this growing sub-class.

Available at: arXiv:1411.2542


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